C Language Declarations Using a Global Variable
Example
Use of global variables is generally discouraged. It makes your program more difficult to understand, and harder to debug. But sometimes using a global variable is acceptable.
global.h
#ifndef GLOBAL_DOT_H /* This is an "include guard" */
#define GLOBAL_DOT_H
/**
* This tells the compiler that g_myglobal exists somewhere.
* Without "extern", this would create a new variable named
* g_myglobal in _every file_ that included it. Don't miss this!
*/
extern int g_myglobal; /* _Declare_ g_myglobal, that is promise it will be _defined_ by
* some module. */
#endif /* GLOBAL_DOT_H */
global.c
#include "global.h" /* Always include the header file that declares something
* in the C file that defines it. This makes sure that the
* declaration and definition are always in-sync.
*/
int g_myglobal; /* _Define_ my_global. As living in global scope it gets initialised to 0
* on program start-up. */
main.c
#include "global.h"
int main(void)
{
g_myglobal = 42;
return 0;
}
See also How do I use extern to share variables between source files?
